The North American XB-21, also known by the manufacturer's model designation NA-21, and sometimes referred to by the name "Dragon",[2] was a prototype bomber aircraft developed by North American Aviation in the late 1930s, for evaluation by the United States Army Air Corps. Evaluated against the Douglas B-18 Bolo, it was found to be considerably more expensive than the rival aircraft, and despite the ordering of a small number of evaluation aircraft, only the prototype was ever built.

Flown by a crew of six to eight men,[1] the XB-21 featured a remarkably strong defensive armament for the time,[4] including as many as five .30-calibre M1919 machine guns.[1] These were planned to be fitted in hydraulically powered[6] nose and dorsal turrets, in addition to manually operated weapons installed in waist and ventral positions.[4] Up to 10,000 pounds (4,500 kg) of bombs could be carried in an internal bomb bay, with 2,200 pounds (1,000 kg) of bombs being able to be carried over a range of 1,900 miles (3,100 km).[4]

Undertaking its maiden flight on 22 December 1936 at Mines Field, company test flying indicated a number of minor problems.[7] Modifications resolving these resulted in the aircraft being re-designated NA-39, and, accepted by the U.S. Army Air Corps as the XB-21. The aircraft, which had been assigned the serial number 38-485,[1] was evaluated early the following year in competition against a similar design by Douglas Aircraft, an improved version of the company's successful B-18 Bolo.[1]

During the course of the fly-off, the gun turrets proved troublesome, their drive motors proving to be underpowered, and issues with wind blast through the gun slots were also encountered.[6] As a result of these problems, the XB-21's nose turret was faired over, while the dorsal turret was removed.[1]

The XB-21 proved to have superior performance over its competitor,[7] but price became the primary factor distinguishing the Bolo and the XB-21.[5] On this account, the modified B-18 was declared the winner of the competition, Douglas quoting a price per aircraft of $64,000 USD, while North American's estimate was $122,000 USD per aircraft,[1] and an order was placed for 177 of the Douglas aircraft, to be designated B-18A.[5]

Despite this, the Army Air Corps found the performance of the XB-21 to have been favorable enough to order five pre-production aircraft, to be designated YB-21.[1] However, soon after this contract was awarded, it was cancelled, and none of the YB-21s were ever built, leaving the XB-21 as the sole example of the type ever constructed.[1] Operated by North American Aviation, the XB-21 served as a research aircraft until its retirement.[3]

Although the XB-21 failed to win a production contract, it was the first of a long line of North American Aviation medium bomber aircraft, and provided experience and knowledge that assisted in the development of the North American NA-40,[8] which, developed into the B-25 Mitchell, would become one of the Army's standard medium bombers of World War II.[9]